1. Field of the Invention
The present invention relates to a rubber composition for a tire tread. More specifically, it relates to a rubber composition for a tire tread obtained by blending an A-B type block copolymer composed of (A) a poly(styrene-butadiene) block or a polybutadiene block, and (B) a polyisoprene block, as a compatibilizing agent, into a rubber composed of natural rubber (NR) and/or polyisoprene rubber (IR) and a high cis and high linearity polybutadiene rubber (BR) so as to improve the abrasion resistance and chipping resistance thereof, without impairing the processability thereof.
2. Description of the Related Art
In the cap treads of large sized tires for trucks, buses, etc., it has been known in the past to blend in butadiene so as to improve the abrasion resistance. Further, for the butadiene rubber, it has been known that blending of butadiene rubber having a large cis component or butadiene rubber having a large linearity (or melt viscosity) has a large effect in the improvement of the abrasion resistance. However, if a plurality of polymers are blended and used as tire tread rubber compositions, etc., a problem of phase separated interfaces arises when the polymers are incompatible to each other. In most cases, this has a detrimental effect on the tensile strength, tear strength, abrasion resistance, etc. With respect to the problem of decrease in the breaking characteristics due to the above-mentioned incompatibility of the polymer blend, the incorporation of a small amount of a block copolymer composed of polybutadiene and polyisoprene into a blend of natural rubber/polybutadiene rubber is described in J. Apply. Polym. Sci., 49 (1983) and RCT. 66 (1993). However, these references only consider the above specific block copolymers in a limited fashion and do not describe any common studies or opinions as to the industrially important polymers containing SBR blocks having a large degree of freedom.
The present inventors previously provided a rubber composition for a tire tread having improved abrasion resistance, heat buildup, and wet grip by blending a high cis polybutadiene rubber and medium vinyl polybutadiene rubber into natural rubber and/or polyisoprene (Japanese Unexamined Patent Publication (Kokai) No. 1-90232) and also provided a rubber composition having improved breaking characteristics by incorporating not more than 20 parts by weight of a specific A-B block copolymer, as a compatibilizing agent, into a blend of natural rubber and/or polyisoprene and high cis polybutadiene rubber (Japanese Unexamined Patent Publication (Kokai) No. 8-193145). However, even in these rubber compositions for treads, there are still problems in the processability at the time of mixing such as kneading performance of mixing and the chipping resistance was not satisfactory either.